Chalcogen substitution and ionization effects on the fluorescence behavior of 8-chalcogen-quinoline-BODIPY: a DFT and TD-DFT investigation

Chalcogen substitution and ionization provide effective routes for tuning the photophysical properties of BODIPY-based fluorophores, yet their combined impact on excited-state proton and charge transfer remains insufficiently understood. Herein, density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations are employed to systematically investigate the effects of chalcogen replacement (O, S, Se) and protonation/deprotonation on the electronic structure and fluorescence behavior of 8-chalcogen-quinoline-BODIPY (8QBDY-X, X = O, S, Se) derivatives. The results reveal that heavier chalcogens enhance π-delocalization and excited-state polarizability while weakening the ground-state intramolecular hydrogen bond and facilitating excited-state intramolecular proton transfer. Frontier molecular orbital, molecular electrostatic potential, dipole moment, Wiberg bond index, and NBO/AIM analyses consistently demonstrate that protonation stabilizes localized excited states with large electronic and optical gaps, whereas deprotonation narrows these gaps and activates photoinduced electron transfer or intramolecular charge transfer pathways. Optical band gap and exciton binding energy analyses further show reduced electron-hole binding in the deprotonated sulfur- and selenium-substituted derivatives, rationalizing their red-shifted and tunable emission. Notably, the computed excited-state characteristics of 8QBDY-O are qualitatively consistent with the experimentally observed ON/OFF fluorescence switching upon protonation/deprotonation. These findings provide mechanistic insight and design guidelines for BODIPY-based fluorescent sensors.